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Full text of "Text Book Of Mechanical Engineering"

IOO2

Appendix V.

The inlet valves  of the engine are automatically  opened

against springs, though lever-opened valves are now much in

favour; and the exhaust valves are lifted by a cam driven*by a

1:2   gear from the crank shaft (see Fig. 878, p. 915).    Ignition

has been fully described at p. 963, but a very interesting discovery

has been accidentally made by Panhard's workmen : that a break

in the secondary circuit, outside the engine, of about i m/m, will

cause the spark to act with greater certainty, even jumping across

dirty inside terminals.      In  smaller  un-governed  engines, the

advance spark (Fig. 898, p. 964) is adopted to regulate the speed

by hand, but in the larger engines a centrifugal governor is placed

on'the second motion or cam shaft, which formerly closed the

exhaust valve to lower the speed, but now throttles the mixture,

In most cases it is permitted to put the governor out of gear so

as to quicken the engine by advance spark or other method.

The speeds adopted are higher than for land engines, for this is

the only means of reducing engine weight.    They vary from 650

to 2000 revs, per m., though few rise to 2000;  some makers,

running at 650 to 850 with governor, obtaining the higher speeds

by hand.    Stroke and diameter are often about equal, and thus a

simple but rough rule at 1500 revs, is

B.H.P. = *

10

The divisor 10 becoming 20 at 750 revs., and 15 at 1000 revs.:
or for any speed, divisor = 15,000 — revs. If stroke and diameter
are not equal, d*s may be substituted for dz.

The full revolutions transmitted along shaft G, have to be
suitably reduced by change wheels at j in the gear box g> giving
three forward speeds as required, on shaft H, with a reverse
motion through the intermediate wheel K; the changes being
effected by the 'clutch handle L and rod M, which slide the wheels
along their shafts, a method which causes much wear, but is
simple, and has not been superseded. Thrust bearings are pro-
vided at K and v. The motion is carried forward by bevel gear
T to the clutch pinion shaft /, passing through the compen-
sating or differential gear in box TJ, already described at p. 526,
which serves to disunite chain pinions w w, and permit their